How to fix the error message - 

The operator '<' can't be unconditionally invoked because the receiver can't be 'null' Try adding null check. Screenshot and  original code listed below. When I added null check if (invalue?.length! <10 ) did not resolve the issue.

 

validator: (String? inValue) {
if (inValue?.length < 10) {
return "Password must be >=10 in length";
}
return null;
},

Thanks to everyone answering my previous question. Decide to put the entire flutter widget program here. Using input from I got from the experts here. 

Major Issues found -

keyboardType: Flutter complains The named parameter keyboardType isn't defined.
How do I define it here?

if (inValue.length ==0) Flutter says use isempty
So I replaced like this if (inValue.isEmpty) but then Flutter shows the following
The property "isEmpty" can't be unconditionally accessed the because the receiver can be null
Try making the access conditional (using?.) or adding a null check to the target('!')

Also the onsave feature isn't working. There are others too many too list

The entire code is listed below. This code was written in a book in 2019.

import 'package:flutter/material.dart';

void main() {
runApp(const MyApp());
}

class MyApp extends StatefulWidget {
const MyApp({super.key});

@override
_MyApp createState() => _MyApp();
}



class LoginData {
String username = "";
String password = "";

}

class _MyApp extends State {

LoginData loginData = LoginData();
final GlobalKey<FormState> _formkey = GlobalKey<FormState>();

@override
Widget build(BuildContext inContext) {
return MaterialApp(home: Scaffold(
body : Container(
padding : const EdgeInsets.all(50.0),
child : Form(
key: _formkey,
child: Column(
children: [
TextFormField(
keyboardType:
TextInputType.emailAddress,
validator: (String? inValue) {
if (inValue.length ==0) {
{//do something
return "Please enter username";}
}
return null;

};
},
onsaved: (String inValue) {
loginData.username = inValue;
},
decoration: const InputDecoration(
hintText: "[email protected]",
labelText: "Username (eMail address)"
)
),
TextFormField(
obscureText: true,
validator: (String inValue)
if (inValue.length < 10) {
return "Password must be >=10 in length";
}
return null;
},
onSaved: (String inValue) {
this.loginData.password = inValue;
},
decoration: InputDecoration(
hintText : "Password"
labelText : "Password"
)
),
ElevatedButton(
child : Text("Log In!"),
onPressed: (){
if (_formkey.currentState.validate()){
_formkey.currentState.save();
print ("Username: ${_loginData.username}");
print ("Password: $_loginData.password}");
}
}
)
]
)
)
)
));
}
}

Thanks to everyone for there help on my previous question.

Using Flutter ver 3.3.8 and Android Studio Dolphin | 2021.3.1 Patch 1 (This is the latest version that i'm using. Since the book is using an older version.)

Modified the Widget with this code see below

Still having issues with this line of code -

if (inValue?.isEmpty) {
return "Please enter username";
}

Flutter shows this msg - A nullable expression can't be used as a condition. Try checking that the value isn't 'null' before using it as a condition. See screenshot of the message listed below. A;so put the full code below the screenshot.

import 'package:flutter/material.dart';

void main() {

runApp(const MyApp());

}



class MyApp extends StatefulWidget {

const MyApp({super.key});

@override

_MyApp createState() => _MyApp();

}


class LoginData {

String username = "";

String password = "";



}



class _MyApp extends State {



LoginData loginData = LoginData();

final GlobalKey<FormState> _formkey = GlobalKey<FormState>();



@override

Widget build(BuildContext inContext) {

return MaterialApp(home: Scaffold(

body : Container(

padding : const EdgeInsets.all(50.0),

child : Form(

key: _formkey,

child: Column(

children: [

TextFormField(

keyboardType: TextInputType.emailAddress,

validator: (String? inValue) {

if (inValue?.isEmpty) {
return "Please enter username";
}

return null;

},
onSaved: (String inValue) {
loginData.username = inValue;
},

decoration: const InputDecoration(
hintText: "[email protected]",
labelText: "Username (eMail address)"
),
),

TextFormField(

obscureText: true,

validator: (String inValue) {
if (inValue.length < 10) {
return "Password must be >=10 in length";
}
return null;
},

onSaved: (String inValue) {
this.loginData.password = inValue;
},

decoration: InputDecoration(
hintText : "Password",
labelText : "Password"
),
),

ElevatedButton(

child : Text("Log In!"),

onPressed: (){

if (_formkey.currentState.validate()){

_formkey.currentState.save();

print ("Username: ${_loginData.username}");

print ("Password: ${_loginData.password}");

}

},

),

]

),

),

),

));

}

}

Getting this error message in Flutter The argument type 'String? Function(String)' can't be assigned to the parameter type 'String?)?'.

For the line listed below - 

if (inValue.length == 0)

How do I resolve it?